1. Field of the Invention
The invention relates to a method of treating strengthened glass to maintain the strength of the glass, to strengthened glass components so treated, to laminated glazings comprising such components, and to a method of bonding a layer of polyurethane to glass.
2. Description of the Prior Art
It has previously been proposed to enhance the strength of glass by etching the glass surface to remove or reduce flaws therein which might serve as a site for the initiation of cracks in the glass. This has been done by acid etching or polishing which removes glass stock by dissolving away the glass, so either removing the flaws or at least smoothing the bottoms of the flaws thereby reducing the stress concentrating effect caused by the flaws in glass subjected to bending forces.
Thus it has previously been proposed, for example in Swiss patent specification 367 942 dated 1958 and U.S. Pat. No. 3,711,263 dated 1969, to enhance the strength of glass by etching the surface up to a depth of 80 or 20 microns respectively to remove surface defects and thereby strengthen the glass; the patent specifications suggest applying silica or an inorganic layer over the etched surface as a protective or hard layer. According to the Swiss patent specification, optimal strengthening may be achieved by thermal treatment of the glass directly after etching and before provision of a protective layer. According to the U.S. patent the glass treated is chemically toughened before etching.
GB patent specification 1 359 165 dated 1974 relates to laminated glass for use in buildings and vehicles; it teaches subjecting a face of a glass pane to a rectifying treatment which reduces the impairment of that face by faults liable to cause stress concentrations when tensioning forces are applied to the glass pane, and using that pane in a laminate with the face which has been subject to the rectifying treatment located internally of the laminated. The rectifying treatments described include fire polishing and chemical dissolution (etching). Because the etched face is protected within the laminate (the invention is indicated to be especially suitable for production of laminated automotive windscreens), it is used in contact with an interlayer material used in the laminate, typically polyvinylbutyral.
The fact that the strength of glass may be enhanced by etching or other surface treatment to remove or modify surface faults, and that the etched surface may subsequently be protected by coating or used in contact with a thermoplastic layer within a laminate has thus been documented for many years. Despite this, and the fact that there has been a continuing need for glass components with a higher strength to weight ratio--especially for use in the construction of aircraft windshields--the only commercial use of surface etching to increase the strength of glass components of which the applicants are aware has been to increase the strength of very thin glass (typically 0.05 to 0.5 mm thick) used for solar cell cover glasses. It is believed that this is because of the difficulty in maintaining the improvement in strength during subsequent processing to assemble the glass into a window.